Bulk material mixers are known which comprise a container with conical bottom and are suitably equipped to allow withdrawal of substreams of bulk material at various levels with these substreams subsequently combined again.
Such bulk material mixers can be used as gravity mixers or circulation mixers. For example, the German publication DE-AS 17 57 540 discloses a gravity mixer which includes a plurality of vertical pipes provided with a number of lateral openings to allow the withdrawal of substreams of bulk material at different levels. With their lower end, the pipes project through the conical bottom of the container and converge into a common discharge pipe.
The German publication DE-OS 32 08 499 discloses a container which is usable as gravity mixer as well as circulation mixer. The container accommodates a plurality of like funnels which are arranged one above the other with the inlet of each funnel being approximately at the same level as the outlet of the next higher funnel.
The known prior art requires, however, a relatively long mixing time and provides a mixing quality which is not yet satisfactory.